The present invention relates generally to apparatus for providing a measure of the distance between the apparatus and an object. In particular, the present invention is directed to automatic focusing systems in which a primary optical means such as the taking lens of a camera, is moved to maintain an image of the object in focus at the plane of a photographic film.
One highly advantageous type of automatic focus apparatus is the spatial image correlation type. Examples of different forms of arrangements of this type can be found in U.S. Pat. Nos. 3,836,772; 3,838,275; 3,958,117; 4,002,899; and 4,103,152 by Norman L. Stauffer and assigned to the same assignee as the present invention and in U.S. Pat. No. 3,274,914 by K. Bedermann et al.
In my U.S. Pat. No. 4,078,172 issued Mar. 7, 1978 and assigned to the assignee of the present invention, I provide a continuous automatic focus system that produces an operation that occurs only with respect to the position of the major extremum to supply an accurate focus correction signal and to drive the lens in an appropriate direction to achieve proper focus. In my U.S. Pat. No. 4,107,520, issued Aug. 8, 1978, I provide an improved continuous automatic focus system which permits proportional control, that is, driving of the motor to position the lens at relatively high rates for large errors and reducing the speed of the motor drive at smaller errors to prevent overshoot and focus oscillation. In my copending application Ser. No. 834,760 filed Sept. 19, 1977, which is assigned to the assignee of the present invention, I provide several improvements to continuous focus proportional controllers which give superior operation in various ways.
One problem which has been encountered relates to an operation of an auto focus system when the scene being viewed contains too little contrast for the light sensors in the electronics of the system to operate satisfactorily. This often produces a system which oscillates or hunts for a satisfactory focus condition or one which drives to a position where the subject is not in focus. Since the auto focus systems of the prior art operate by photometric comparison of the light falling on two detector arrays, the contrast between the background and the subject may so low that it is difficult to detect the subject with sufficient accuracy. This is particularly true in scenes where there is little horizontal contrast even though there may be good vertical contrast as, for example, in some scenes involving the horizon. The importance of horizontal contrast occurs because the system normally operates by comparing signals which are obtained by rotation of a mirror about a vertical axis so that the scan lines are horizontal. The low contrast problem may also arise during the panning of a movie camera particularly where the primary subject is not in the view of the auto focus system. While in a well aligned prior art system the lower limit of resolvable contrast may be quite low, there is in all cases a necessary lower limit imposed by variations in the match of the optical detectors. In view of this contrast limit, the focus system may make an error in determining the correct focus position under the above-described low contrast conditions.
In my U.S. Pat. No. 4,085,320, one solution to this problem is shown. In that patent, signals produced by detectors receiving radiation from the subject and from the background are compared throughout the range or area of the scene to determine whether there is sufficient difference and when the auto focus mechanism is activated. While the circuit of U.S. Pat. No. 4,085,320 operates satisfactorily for most situations, a difficulty may be encountered when fluctuation in the lighting occurs. For example, with flourescent lighting the variations in the correlation signal may cause the activation of the auto focus circuits even with insufficient contrast.